Plant gossips… are almost cotton!

The hard life of a pollen grain

Pollen grains are fabulous. Sure, I tell you. Well, OK, some of them make people allergic but hey only some of them, not all of them. Others are mostly harmless…

Moreover, they are beautiful. Believe it or not, here is some evidence: just have a look at this one. Deliciously spiny, just enough not to look too common. But they are still common, from spring to fall, springing and falling all over everywhere. This one is coming from a beautiful plant, Commelina communis, a garden escapee that acclimated the East Coast that is colonizing here and there urban landspace, spreading a light blue flower that’s not without reminding me of the eponymous book from Raymond Queneau, and Le Chiendent visions as well.

But let’s not forget about pollen grains. That is, the quasi-gamete of plants. Actually a real organism per se, proudly representing a gametophytic generation whose function is to produce sperms in plants. Alas, the meandering paths of evolution reduced it to such a specialized organ that it’s nowadays brought down to an almost unicell structure with a caring nucleus (called vegetative though it’s not dormant at all, but we’re speaking about plants after all), housekeeping two gametes awaiting as nuclei, gently packed during their journey up to an unfertilized ovule. Just like almost any male gamete, pollen is produced in huge quantities, and that’s where its real problem lies. Of course, you need a huge crew to fertilize ovules all around the landscape, given that many pollen grains will fall apart at the wrong place, from insect legs and stomachs to stigmas of other species…

Let’s be fair, lots of them succeed into their air flight journey, and they end up onto conspecific flowers. This is what this post is all about. Landing to such a place is unfortunately a dramatic experience too.

In a receptive flower, only the top of the stigma is sticky enough to retain welcomed guests. Beyond this area, this is just a cliffstand harmful place to fall. There isn’t much hope out of stigma’s head. These two little guys can tell you: there’s no way to stick anywhere, “camping is not safe at that place”. Kind of edge of the world…

But then, to the fortunate stickers, this is only the beginning. No time to lose! You can rehydrate yourself safely after this first huge leap, but there you go…

… Yep, the pollen grain is not alone. Brothers from yesterday already turn into wild competitors (even in naturally selfing species). The big race begins. One should germinate as fast as possible. One has to fertilize the host’s ovules. First arrived will be served. And in Commelina communis, each flower only has up to four ovules. That’s rather small, as competitors are many: up to fifty onto this stigmate for example. And this is nothing exceptional. Though this might even be a fairly gentle competition, since for you readers candidates once were millions… The thing is that while many animal species manage to limit the number of potential fathers (or at least guys do), plant gametes compete with many unrelated guys most of the time…

So to say, lucky pollen grains still have to face the greatest race ever, without any previous training. They have to germinate. Bring gametes to the ovule. Fast. Therefore, they initiate very quickly the growth of a very strange structure, the pollen tube. No secret, that’s an actual tube, bearing the reproductive nuclei into the depth of the stigma, down to the ovary. If you look at a flower, you’ll see that’s a very long way, particularly if you’re just a few dozens micrometers big at start. Most often for late pollen grains, the way is already blocked by the earlier guys. Metabolic stress is huge, for you have to produce several centimeters of a costly callosic tube (at a speed of a few millimeters an hour). Time is precious so far, paternity of seeds is at stake…

Given the hurry, it doesn’t take a long time before the pollen’s precious reserves are out. There’s just the empty wall left over, anchored onto the stigma by some remnant pollen tube. Some mother plants contribute to feeding tubes halway, though it’s not a necessity and some simply don’t.

This pollen grain probably has a chance: it’s in a good place and doesn’t have that many neighbouring competitors. That’s a good start. We can even see the pollen tube (bottom), gone for the Holy Grail.

This one may have had a chance too, but by a somewhat of a miracle: having slipped over the stigma, its odds were rather small. But it still germinated and went its way through the stigma even if the wall of stigma is usually not the best place to start growing, compared to the top resting place.

This may have been the chance for a real shortcut, precarious but rewarding – a few millimeters can really make the difference…

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